Intermittent P0306 Code on a '96 Continental

[kr98664]

Greetings from a new guy. I recently purchased a '96 Continental which has had a sporadically reoccuring "Service Engine Soon" light. I would appreciate any help from the great minds here. According to my code reader, it is a P0306 fault, meaning misfire detected on #6 cylinder. There are no other codes present. I'm using an Equus 3100 Code Reader which is supposed to be able to retrieve manufacturer specific codes in addition to the generic OBD II items:


Equus 3100 OBD II Code Reader (http://www.iequus.com/3100.asp?catshort=3100)

It is possible there are some other codes present that this reader can't pick up, but I don't know for sure.

This car has a bit of a history. Purchased used from a local dealer, the "Service Engine Soon" light was on and to be repaired under warranty. Expecting something simple like an O2 sensor, etc., imagine everybody's surprise to discover low compression in #6 cylinder. Lots of carbon was found in the combustion chambers, with the result that one of the 4 valves in #6 cylinder failed to seat properly and was burnt. A valve job, fortunately under warranty, took care of the compression problem. The carbon build-up was explained as symptomatic of a powerful engine that spent much of its life loafing. I've made it a point to drive this hot-rod at full throttle every now and then, in the name of science of course.

Shortly after the repairs, the light came back on a few times over the course of several weeks. Each time it was handled promptly under warranty. I know that an O2 sensor (not sure which one) and the EGR back-pressure transducer were changed.

Enough time has passed that the car is no longer under warranty. In addition, I'm stubborn enough to think I can figure this one out with some careful troubleshooting. The shop that performed the work (contracted by the dealer) has been extremely helpful but I hate to wear out my welcome after several months. Other than the O2 sensor and EGR transducer faults which didn't return, they found no other codes (using a Snap-On scanner) besides the P0306.

As far as the infamous EGR passage, I do NOT know if that has been cleaned out yet. The car has 114K miles, so it likely needs it. I do not know if the EGR passage would have been visually inspected when the heads were off. It is entirely possible they were never checked depending how much disassembly was needed during the valve job. I do plan to check and clean this passage but would appreciate further guidance. From what I understand, I'd have other codes besides the P0306 if this passage was entirely at fault.

The symptoms include a very light chugging in the 1500-2000 RPM range with the throttle just off-idle. This is sporadic and does not always happen. There is no problem under heavy acceleration, only off-idle in that RPM range when under a light load. The tach needle stays steady when the problem happens. When coasting, climbing a slight upgrade (or steeper) or above 2000 RPM, the chugging promptly quits. There is a definate change precisely at 2000 RPM. The chugging feels almost like a torque converter abruptly locking/unlocking or transmission hunting between gears. (Shifting out of overdrive didn't help) It feels a little more pronounced than a single cylinder misfiring, which I doubt I'd be able to feel. Occasionally, the idle is not as smooth as other other times, but I wouldn't call it rough by any means.

According to the factory manual, the only parameter that trips a misfire code is if the crankshaft speed sensor doesn't detect the normal increase in RPM after the power stroke for each respective cylinder. In other words, #6 cylinder isn't doing its share of the work compared with the others. This condition must be detected on two consecutive legs before the light comes on, to avoid nuisance indications.

I've been following the factory manual, but am a bit handicapped without a $2000 NGS scanner/tester. I've tried to extrapolate the various corrective actions based on what I can deduce, which is possibly leading me astray. The most likely culprit, based on a P0306 code by itself, was a bad fuel injector. The shop also thought this was the most likely culprit should the problem return. Since the problem was so intermittent, I swapped #6 and #8 injectors for troubleshooting. The code did NOT follow the injector, hopefully ruling out a set of injectors. The inlet screen was clean on each injector and I've been careful to only run 92 octance Texaco fuel. Presently I lack a tester to check the fuel pressure, but would think this was not the problem based on a fault for a single cylinder.

The manual also pointed to the ignition system. Interestingly, the ignition system uses a double-strike pattern in certain conditions under 2000 RPM, which seems to jive with the chugging I've experienced. As previously mentioned, there is an abrupt end to the chugging precisely at 2000 RPM. Without being able to test the hi-tension waveform, I swapped the two coil packs and still had the P0306 code. New plugs were installed with the valve job. I confirmed the correct effectivity and swapped plugs #5 and #6, still no change. I also found #6 plug wire misrouted parallel to #5. Rerouting that plug wire per the manual's illustration was no help, either. Unfortunately, each plug wire is a specific length so I can't swap them for troubleshooting. The plug wires have a 1996 date code, making them suspect. I did try checking for broken down insulation with the water spray method but no change.

Sorry for setting a forum record for the longest post. I don't mind spending money for a definitive fix but am averse to throwing parts at a problem until it hopefully goes away. Since the problem is so intermittent, I was hoping to be able to track it down with some careful troubleshooting. At this point, my plans are to inspect/clean the EGR passage, double-check compression and possibly a set of plug wires. Would anybody recommend a new EGR valve while I'm at it? I also have the procedure to check whether or not the EGR back-pressure transducer is seeing normal values.

Once again, sorry to have babbled on so long. I like to make sure all information is available to anybody interested. I couldn't find anything similar in the archives. I'll let everybody know what happens after inspecting the EGR passage. Perhaps there are two separate problems, i.e. a bad plug wire causing the misfire code, and a restricted EGR system that causes the chugging but isn't quite bad enough to set a separate code.

Thanks in advance for any suggestions,

Karl w/ a '96 Continental
Vancouver, WA

[redlineauto]

replace the wires. They are due any way. The boots have a tendancy to go bad and arc out where you can't see them.

You might want to put a vacuum gauge on the engine just to rule out the valve train again.

[DaKat]

Great first post
Here's a ^bump^ for ya' smile.gif

[kr98664]

Okay, I'm going to spring for a set of plug wires. The more I think about, yes, they are due for replacement regardless of whether or not they are not the cause of the problem.

Any suggestions where to get a set? From painful experience, I'm partial to OEM or top quality aftermarket brands. Several local dealers had them in the $150+ range. NAPA had them for around $110. FordPartsOnline.com has OEM plug wires for about $80 with shipping, which is an excellent price. (It would take at least a week to get them from FordPartsOnline due to the weekend) Are there any comparable aftermarket brands I might be able to find locally? I haven't checked any of the discount chains because of past lessons learned the hard way, but would consider it if I knew certain brands available anywhere (Champion, Bosch, etc.) were okay.

I have a Scotsman's wallet (Velcro flap screams when opened) so want to make sure my money is well spent.

Thanks for any comments.

Karl w/ a '96 Continental
Vancouver, WA

[kr98664]

Hey Boys and Girls, cautious success to report. The new plug wires arrived and were installed this morning. (Great service from FordPartsOnline.com, BTW) Here's something curious I noticed when I removed the old plug wires. The boot on #6 (the suspect cylinder) had several drops of water on the outside. This is on the long shank that fits down inside the valve cover. I'm a bit mystified, but think this would have been condensation. All of the other plug wires were dry. The car had just been driven about 20 miles in the rain after sitting outside overnight (yep, more rain), but I can't see how rain water would have got inside there. The cover over the plug wire raceway was installed, and there were no traces of water anywhere else under the hood. I can only figure it was condensation and that plug wire had enough of a fault that it didn't heat up like the others to burn off the moisture. I may be out in left field here, but it was definately wet while the others were dry.

Resistance values end-to-end on the old plug wires checked fine, but that doesn't rule out a breakdown in the insulation.

BTW, the light had been coming on more frequently in the past week, about once a day. The light was cleared yesterday, and came back during an early morning trip before changing the plug wires. (Still P0306 code only) The chugging problem had become noticeably stronger, too, occuring more often than not under the conditions previously mentioned.

Check out what else I found. I was preparing to follow the rest of the troubleshooting flowcharts in the factory manual, if the problem continued after changing the plug wires. Before going out for a test drive, I was poking my head under the hood, looking at the items I planned to test. On the EGR Differential Pressure Transducer, I found one of the sensing lines had a small split at the end. Eureka!!! This was on the intake manifold side of the EGR tube orifice. With this line split, fresh air could be drawn into the intake manifold when EGR flow was commanded. Instead of inert exhaust gas as designed, fresh air full of oxygen was being drawn in from the pressure transducer connection. This would cause leaning of the fuel/air mixture.

This hose is very short, only about 2 or 3 inches, with a 45 degree bend. It appears to be heat resistant, as is very thick and has a red rubber core. Since the transducer was previously replaced by the shop, I'm wondering if this hose may have also been replaced at the same time. The existing piece looks like generic tubing cut to length and forced around the bend.

Can anybody else with a '95-'97 Continental be so kind as to look under their hood for me? I'm pretty sure this should have been a preformed piece due to the bend in the middle of such a short length. The transducer is easy to see with just a flashlight, no disassembly of anything is required. Please look at the driver's side end of the aft cylinder head. The transducer is a box about the size of a deck of cards, with two hose connections on the bottom and a three-wire electrical plug. The two hoses run down to fittings on the side of a steel line about 3/4" in diameter. The hose in front is the one with the bend. It split at the end from being forced around the corner. I was able to reposition/seal it for right now until I can find out whether or not I need a preformed line from the dealer. If not, I at least plan to replace it with new heat-resistant tubing and put clamps on both ends to prevent splitting in the future. Unfortunately, I ran out of time for this.

What a difference on the test drive. The chugging is gone, or nearly so. It may be my imagination that I still felt a very slight chugging, but nothing like before. I ran out of time for a long test drive to make sure the light didn't return, but so far so good. I'll let you know if the light returns in the future but I hope it is gone.

At this point, it looks like I had two problems. The split hose caused the chugging via the EGR system, and the plug wire was at fault for the P0306 code. I would like to put some more miles on it before saying it was a definitive fix, but so far so good.

In the meantime, could somebody please compare that line I mentioned? I'd like to attend to that right away and need to know if it must come from the dealer. Thanks for the help.

Karl, with a '96 Continental that runs much better now.

[ December 21, 2002: Message edited by: kr98664 ]

[kr98664]

The saga continues...

No hard fix yet, but much improvement. The problem returned after the repair of the split hose as previously mentioned. I'm still getting the same chugging, although much less noticeable, and the occasional P0306 code.

Much to my delight, I found the EGR valve not fully seating. I determined this by following the factory Emissions Diagnosis manual. Even without the factory scanner, I was able to read the EGR Differential Pressure sensor that monitors EGR flow rate. Exhaust gas flowing to the EGR valve (and then the intake manifold) passes through a restrictor. The Differential Pressure sensor compares the pressure before and after the restrictor. With no flow, the pressures are the same. With high flow, the downstream side has a lower pressure. The sensor is a potentiometer with an output voltage proportional to EGR flow. By backprobing the connector, I could read this voltage with a digital voltmeter and see the same signal that the PCM sees. This seemed pretty exciting to me. The value for no flow was determined with the engine off but key on, becoming the reference point value.

Per the manual, I found flow intermittently when the valve was commanded closed. The only time this happened was after revving the engine. When the valve didn't seat fully, the engine would idle less smoothly and I'd get the P0306 code within a couple of minutes. The manual had me disconnect/plug the vacuum line that actuates the EGR valve, but the flow continued. Rather than screw around cleaning the valve, I gladly plunked down $58 at the dealer for a new valve. I had a hard fault and was ready to take care of it.

The EGR flow at idle must have been just under the threshold to set a code. The idle quality was barely affected. The EGR monitor within the OBD II system does check for flow at idle, and insufficient flow when commanded on. However, from what I understand, the OBD II system never checks for excessive flow when commanded on, which is what I may be experiencing.

Much to my dismay, the problem has still continued, although to a lesser degree. With the holidays, time to work on the car has been a scarce commodity. So far I have not had time to confirm that the new EGR valve closes fully when commanded. At this point, I'm suspicious of the control solenoid that routes vacumm to the EGR valve when commanded by the PCM. My best guess is that this solenoid is intermittently hanging up and not venting the EGR valve actuator line fully when commanded, letting the valve hang open slightly. I'm also wondering if operating slightly open like this could have led to the demise of the old valve due to carbon buildup. I have a new solenoid on order but of course am pacing like crazy waiting for it to arrive.

I had the brilliant idea to block off the EGR passage for troubleshooting. A simple plate was fitted between the EGR valve and the intake manifold. I was busy patting myself on the back, figuring the chugging would be gone and I'd know for sure the EGR system was at fault so it could be repaired properly. Alas, it made little difference and might have even been a little worse. Reading through the manual, it appears that ignition timing and injector scheduling is also changed when EGR flow is commanded. I'm hoping this is why nothing much changed. Since this block-off plate was not part of the factory troubleshooting, I'm not sure how valid this technique may be. It could also point to something like a vacuum leak, but I just don't know at this point. I need to install the new solenoid and confirm its proper operation before getting too bogged down with anything else.

While waiting for the new solenoid, I'm planning to rule out other potential problems. The manual makes frequent mention of proper fuel pressure, so I've picked up a fuel pressure tester. I also need to rule out vacuum leaks, and have posed a question on that subject in another thread.

If anybody has any suggestions, please feel free to jump in. I feel like I'm making progress but wonder if I'm missing something obvious.

Karl w/ a '96 Continental

[ December 27, 2002: Message edited by: kr98664 ]

[FLM]

Hi Karl

I have not worked on the engine you have, but I am enjoying your thorough Engineering based approach to it.
I can offer a few comments, that's about it.
It takes very little EGR flow to adversely affect idle quality. Whenever I crack an EGR valve open at idle (like putting a hand vacuum pump on it) very little vacuum is needed to adversely affect idle quality, and just a bit more kills the engine.
Good idea on monitoring the Differential Pressure sensor, really shouldn't see anything until upwards of 1500 RPM or so, I would think. The manual may mention the minimum RPM for EGR to be enabled.
In the other thread you mentioned IAC valve. Usually if you unplug a Ford IAC at idle, the valve closes, the throttle bypass air is interupted, and the engine dies. Your idea to monitor the operating average control voltage to the IAC while in operation should work. But I wouldn't be surprised to see it moving some up and down on its own when everything is fine. A computer closing the feedback loop on a dynamic mechanical system, air through to crankshaft RPM, not a 100% stable equation.

[ December 27, 2002: Message edited by: FLM ]

[kr98664]

Exciting news to report, for anybody not yet bored to tears by the sordid tales of a man and his code reader.

With a few precious moments to work on the car, I verified fuel pressure is exactly within specifications. From there, I started staring at the various vacuum lines attached to the intake manifold. I unplugged each connector from the manifold and tried to draw air through them with a Mity-Vac vacuum pump.

The suspect EGR regulator solenoid failed big time. It was letting air be drawn into the intake manifold, of course downstream of the MAF sensor. The solenoid probably couldn't do its job of controlling the EGR valve and was also leaking air into the manifold. I have a new one on order, so I plugged the line for troubleshooting right now.

So far I've uncovered three hard faults. The EGR Differential Pressure Sensor (measures EGR flow) had a split sense line. The EGR valve itself wasn't fully closing on occasion, even with the vacuum actuating line disconnected. (The EGR valve is spring-loaded closed) And now I've found the EGR regulator solenoid leaking in a major way.

But wait, there's more! The large vacuum line that heads aft towards the firewall also wouldn't hold pressure. This is a large hose, about 1/2" diameter. At the firewall, it tees to the heater/AC system, power brake booster, parking brake release and canister purge valve. By plugging the lines one at a time, I was able to determine the canister purge valve was also leaking profusely. The line in question supplied the muscle force to move the diaphragm inside the valve. This is a seperate line from the one that feeds canister fumes right beneath the throttle body plates when commanded. I'm not sure, but depending on the what failed inside the valve, fuel fumes may have been fed into the manifold at all times, right near #2 and #6 cylinders. I'm hoping this extra fuel was the cause of the P0306 misfire code.

With two major vacuum leaks plugged, there was a marked improvement on a test drive. I have both parts on order and still need to clear the KAM (Keep Alive Memory) to let the PCM relearn the engine's characteristics without these pesky vacuum leaks. I will do that after installing the new parts and will keep driving to a minimum until then.

For anybody else troubleshooting vacuum leaks like this, there is one word of caution. When I tried to draw a vacuum on the line that fed the heater/AC system, the gauge didn't show any vacuum increase for a long time. It took me a while to realize why. With all the various door actuators in the heater/AC system, that is a lot of volume in the system. With the little hand-held pump, it took quite a while to build up a vacuum. Don't let that fool you.

Karl w/ a '96 Continental